Method and apparatus for stimulating the flow of oil wells

ABSTRACT

A method for stimulating the flow of oil wells consisting of sealing off a portion of the well shaft within the oil-bearing formation of the ground, introducing a combustible fuel not requiring additional oxygen for its combustion into the sealedoff portion of the well shaft and igniting said fuel; and an apparatus capable of performing said method. The combustion produces heat, which tends to melt or burn out sludges, tars, or the like which may be clogging the oil flow passages of the formation, and also produces sudden large quantities of gaseous products of combustion, in a manner similar to a low-order explosion, which fractures the formation to form new flow passages. The flow rate of the fuel may be gradually increased, and rendered intermittent to produce a series of &#39;&#39;&#39;&#39;explosions,&#39;&#39;&#39;&#39; whereby the zone of the oil-bearing formation treated is gradually increased.

U lllletl mates resent [151 3,674,093

Reese July 4, 1972 54] METHOD AND APPARATUS FOR 3,298,439 l/1967 Rees..l66/59 STIMULATING THE FLOW OF OIL 3,313,234 4/1967 Mohauptw WELLS3,339,635 9/1967 Brandon ..l66/59 X [72] Inventor: Dale C. Reese, 2319Meadow Lane, Primary ExaminerStephen J. Novosad Salina, Kans. 67401Att0rneyJohn Arl-lamilton [22] Filed: June 24, 1970 g ['57] ABSTRACT[21] Appl A method for stimulating the flow of oil wells consisting ofsealing off a portion of the well shaft within the oil-bearing for- 52us. Cl ...166/299, 166/63, 166/308 mation of the ground, introducing acombustible fud not [51] Int. Cl ..E2lb 43/26 requiring additionalOxygen for its combustion into the Sealed- [58] Field of Search..166/299, 302,303, 308,57, Off Portion of the shaft and igniting Saidfuel; and an 9- 166/59, 63, 222; 175/1 paratus capable of performingsaid method. The combustion produces heat, which tends to melt or burnout sludges, tars, [56] References Cited or the like which may beclogging the oil flow passages of the formation, and also producessudden large quantities of gase- UNITED STATES PATENTS ous products ofcombustion, in a manner similar to a loworder explosion, which fracturesthe formation to form new 3,422,760 H1969 Mohaupt ..l66/63 X flowpassages The flow rate f the fuel may be gradually i 2,732,016 1/1956MacLeod 166/ creased, and rendered intermittent to produce a series ofex- 1449,420 3/1923 8 61 plosions, whereby the zone of the oil-bearingformation 1,843,002 l/l932 Small ..l66/63 X treated is graduanyincreased 2,668,592 2/1954 Piros et al. .....l66/59 X 2,776,816 H1 957Jackson ..l66/59 X 8 Claims, 2 Drawing Figures V, 4 lIIIII/IIIIIII/I/I/I/III/I/IJ METHOD AND APPARATUS FOR STKMULATING THEFLOW OF OIL WELLS This invention relates to new and useful improvementsin oil well treating methods and apparatus, and has particular referenceto methods and apparatus for reviving or improving the flow of oil wellsin which the oil-bearing formation has lost its permeability to the flowof oil therethrough to the well shaft, or in which said permeability hasbeen severely impaired, or in which the permeability is naturally low.

It is well known that oil-bearing formations may lose theirpermeability, so that the rate at which oil can flow therethrough to awell shaft is severely reduced, while there are still large quantitiesof available oil in the formation. This loss of permeability may resultfrom many causes, the principal of which are believed to be as follows:e

1. Chemical damage. Acids and other chemicals with which wells aresometimes treated, usually with the purpose of improving flow, sometimeshave an eventual reverse effect, tending to combine with the oil toproduce thick, gummy emulsions, acid sludges, and bacterial growthswhich tend to obstruct and clog the flow passages of the formation,which is usually a porous body of dolomite, limestone, or the like. Thistype of clogging may extend considerable distances from the well shaft,and is not particularly adapted for removal by the application of heat,since at ordinary temperatures the clogging materials may tend not tomelt, but to bake" into solid masses. They may, however, be burned orshrunk at least to some extent if the heat applied is sufficientlyintense.

2. Tars and paraffins. Oil has either an asphalt or paraffin base, andas it flows through the formation it is subjected to adiabatic coolingas it passes flow restrictions. This cooling causes the base asphalt orparaffin fractions to tend to solidify and deposit on the flow passagewalls, eventually restricting or cutting off the flow of oil. Thiscondition is treatable by heat, since the asphalts and paraffins may bemelted by even very slight temperature increases, but often extends togreat distances from the well shaft, into zones very difficult to reachwith heat.

. Expandable clays. Many formations include bentonite or otherexpandable clays, which, in the presence of wateror well-treatingchemicals, tend to expand or swell to greater or lesser extents to closeor obstruct the oil flow passages. This type of clogging also may extendto substantial distances from the well shaft, and is difficult to treatby heat, since the clay will not melt, although here again it may beburned to some extent if the heat is sufficiently intense.

4. Drilling mud. The drill bit itself, as the well is drilled of course,forms large quantities of pulverized materials, which is mixed with adrilling mud slurry of sufficient viscosity to carry the cuttings to theground surface. When drilling is completed, the well should bethoroughly cleaned of such mud, but the operation is difficult and oftenperformed inefficiently, so that the mud collects in the shaft, oraround the well casing, and blocks the flow of oil.

Loss of flow permeability, or naturally low permeability, is mostcommonly treated with hydrochloric acid or other chemicals designed todissolve portions of the formation itself, or by hydraulic fracturing ofthe formation, or by fracturing with liquid explosives such asnitroglycerine. None of these methods are satisfactory to any greatdegree. Acidizing the well too often results in the fonnation of acidsludges, emulsions, and bacterial growths which have an effect reverseto that desired, tending to clog flow passages despite the fact that theacids do tend to destroy part of the formation to create new passages.There is also the danger that the acid'will eat through ground strataadjacent the formation and into nearby water tables, resulting inflooding of the well. Water fracturing is limited in scope due tolimitations in the rate at which water or other liquids can be injected.These limitations result from pressure limitations of the well casing,and from friction losses in the casing. High explosives often causedestruction of the casing, or even collapse thereof above the point ofdetonation, so that other equipment cannot thereafter be lowered throughsaid casing. Also, high explosives may break through ground strataadjacent the formation into adjoining water tables and cause flooding ofthe well.

Accordingly, the principal object of the present invention is theprovision of a treating method and apparatus which is efficient andeffective against all of the described causes of loss of flowpermeability in oil wells, while not being subject to the shortcomingsand disadvantages of prior methods. The method contemplated by thepresent invention involves a combination, generally, of heat andfracturing methods, by the injection and combustion of a special fuel ina sealed-off portion of the well shaft within the oil-bearing formation.The fuel burns rapidly and produces great heat and large quantities ofgases, creating a high pressure, but burns at a slow enough rate that atmost only a low-order explosion is produced. The fuel supply rate ispulsed to provide a continuous series of these low-order explosions orsudden increases of pressure. Each pulse creates pressure sufficient tofracture a thin zone of the formation, first directly adjacent the wellshaft and successively further away as the fracturing opens passagespermitting access of the high pressure gas to more distant portions ofthe formation. The fuel supply rate may also be gradually increased tomaintain the desired pressures as the zone of treatment enlarges. Thefracturing is sufficiently gentle to avoid the formation of largequantities of pulverized debris, but does open new flow passages. Theseflow passages of course form paths for oil to reach the well shaft, butinitially fonn passages permitting deeper penetration of thehigh-pressure gases and heat into the formation. The heat tends to meltor burn such clogging masses of sludges, asphalts, tars, paraffins, orexpanded clays as may be capable of being melted or burned, therebytending to open many of the old, previously clogged passages, andfurther increasing the flow permeability of the formation. With amodification of apparatus, the process may utilize substantially coldgases where desired.

A further object is the provision of an apparatus capable of performingthe above method.

Other objects are simplicity and economy of both the method and theapparatus, and efficiency and dependability of operation.

With these objects in view, as well as other objects which will appearin the course of the specification, reference will be had to theaccompanying drawing, wherein:

FIG. 1 is a fragmentary vertical sectional view of an oil well includingan exemplary apparatus embodying the present apparatus invention, andcapable of performing the present method invention, the apparatus beingshown in partially schematic form, and

FIG. 2 is an enlarged sectional view taken on line ll-ll of FIG. 1.

Like reference numerals apply to similar parts throughout the drawings,and the numeral 2 applies to an oil well shaft, shown fragmentarilyinvertical section, the oil bearing formation, which is a porous formationof dolomite, limestone, or the like, being indicated at 4 and theoverlying earth strata at 6. The shaft is provided with a tubular casing8 which extends downwardly from the ground surface, its lower end eitherresting on the top of formation 4, as shown, or extending downwardlyinto said formation. in the latter case, the portion of the casingwithin formation 4 is perforated for the ingress of oil. A tubular pipestem 10 of smaller diameter than the casing is disposed within saidcasing, extending downwardly from the ground surface and supporting atits lower end a cylindrical combustion chamber 11 coaxial therewith,said stem being affixed to said chamber so as to communicate with theinterior thereof. The lower end of said chamber is closed by an end wall12. A series of discharge nozzle fixtures 14 are affixed in the walls ofchamber 11 so as to direct jets outwardly therefrom as will appear, saidnozzles being spaced equally about the periphery of said chamber andpositioned to direct their jets normally to the chamber axis, orhorizontally. Each nozzle is fitted with a blow-out plug which normallyseals the nozzle, but which can be blown outwardly from the nozzle inresponse to substantial pressure within chamber 11. A very small fuelfeed tube 9 extends coaxially in stem 10 from the lower end thereaboveto a point well above casing packer 36, as will be described, and theannular space between said tubes is filled with a suitable heatinsulating material 13.

Fixed in the lower end of stem 10, in communication with fuel feed tube9, is an injector nozzle 16 adapted to discharge a liquid fuel from saidstem into chamber 11. Interconnected in stem 10 a short distance abovechamber 11 is a springloaded check valve 18 operable to permit onlydownward flow of fuel through the stem, its loading spring 20 being ofsufficient strength to maintain the valve closed against the pressure ofthe column of fuel, or fuel and water, contained in stem 10 thereabove,but to permit said valve to open when a higher pressure is appliedthereto. Valve 18 is also provided with heat insulation as indicated at19. Intermediate valve 18 and injector nozzle 16, tube 9 isinterconnected by means of conduit 22 to an external switch housing 24wherein the pressurized fuel acts against a diaphragm 26 to close anelectric switch 28 to complete the operative electric circuit of anelectric igniter 30 (which may be a spark gap) disposed in chamber 11adjacent nozzle 16. The leads from switch housing 24 to chamber 11 areenclosed in a conduit 32. Electric current for the igniter may besupplied by a small, replaceable battery 34 carried in housing 24.Diaphragm 26 may be set to close switch 28 and actuate the igniterwhenever injector nozzle 16 is injecting fuel into chamber 11.

Stem 10 is sealed in casing 8, above chamber 11, by means of a casingpacker indicated at 36. Such packers are common and well known in theart, and therefore are not here shown in detail. The packer should be ofsufiicient holding power to contain the maximum pressures to bedeveloped in the shaft during the process, as will be described, toprevent stem 10 from being blown upwardly through the casing. Fuel feedtube 11 should extend to a point above casing packer 36.

Above ground level, stem 10 is interconnected to the lower end of aT-fitting 38, the opposite or upper end of said fitting being sealed bya removable plug 40. Water is delivered to the side inlet of saidT-fitting from a suitable source through pipe 42, a pump 44 having anadjustably variable delivery rate and driven by an electric motor 46,pipe 48, a pulsing valve 50, and pipe 52. Pulsing valve 50 includes avalve body 54 having an inlet 56 into which pipe 48 is connected, anoutlet 58 into which pipe 52 is connected, an outlet 60 into which aby-pass conduit 62 is connected, the opposite end of said conduit beinginterconnected into pipe 42 ahead of pump 44, and a slide 64 mountedreciprocally in the valve body and operable as it is reciprocated toconnect inlet 56 alternately with outlets 58 and 60. The valve slide isreciprocated by means of a link 66 pivoted at one end to said slide, asat 68, and pivoted at its opposite end, as at 70, to a disc 72 rotatablydriven by an electric motor 74, eccentrically of the axis of rotation ofsaid disc. Motor 74 may be of variable speed, or intermittentlyoperated, to supply pulses of pressure to stem 10 at any desiredfrequency.

In operation, the liquid fuel used is preferably of the monofuel type,including in and of itself the supply of oxygen necessary for itscombustion, so that no external source of oxygen is required.Alternatively, of course, separate conduits for fuel and a supply ofoxygen to combustion chamber 11 could be used, but this would require adouble plumbing system, and would be less desirable. The fuel is of atype which, while it burns at an extremely high rate when ignited,accompanied by the production of intense heat and large quantities ofgas, nevertheless burns at a rate substantially less than that of ahigh-order explosive, so as to produce at most an explosive effect ofonly a low order. Various types of fuels presently utilized in rocketry,such as hydrogen peroxide or hydrazine, are suitable for use in thismethod and apparatus.

Since the treatment of a single well would seldom if ever require enoughfuel to fill the entire length of stem 10, the

device is preferably prepared for use by removing plug 40 from the upperend of the stem, pouring in sufiicient fuel for the treatment, pushing awiper sealing plug 76 down the stem until it engages the fuel, thenfilling the remainder of the stem with water and reinserting plug 40.Wiper sealing plugs such as used at 76 are common and well-known in theart, and the plug is therefore not here detailed. Motors 46 and 74 arethen set in operation to drive pump 44 and oscillate pulsing valve slide64, whereby to deliver water in a series of pulses to stem 10, forcingwiper plug 76 downwardly to deliver the liquid fuel through valve 18 toejector nozzle 16 in a corresponding series of pulses. By-pass 62permits pump 44 to operate at a substantially constant delivery ratedespite its intermittent delivery to stem 10, since during the lullsbetween delivery pulses to the stem, the pump output is simply by-passedback to the pump intake. Actually, in most cases, it is desirable thatthe pump delivery rate, or more accurately the amount of water deliveredthereby during each pulse be gradually increased as the treatmentprogresses, for reasons to be described. Pump 44 is of a variabledelivery type for this reason.

As the fuel enters combustion chamber 11 through injector nozzle 16,being expanded to mist or vapor form by said nozzle, it is ignited byigniter 30, which as previously described is actuated by the pulses ofpressure in the fuel. The fuel, thus ignited, burns to create intenseheat, quantities of gas and a resultant high pressure in the combustionchamber, dislodging blow-out plugs 15 and forcing the burning gasesoutwardly through nozzles 14 into formation 4 in the form of powerfuljets. The burning rate of the fuel is sufficiently slow that combustion,accompanied by the production of more heat and more gases, is stilloccurring at a substantial distance away from the combustion chamber.The blow-out plugs prevent any oil or other liquid standing in the wellbore, which it often does to substantial depths, from entering chamber11 prior to combustion, which could cause destruction of the chamber dueto the density of the liquid. The supplying of fuel through small tube 9well insulated in stem 10 prevents possible pre-ignition of the fuelwithin said tube by the heat generated in the well bore, the safetybeing provided both by the insulation and also by the fact that due tothe small tube diameter, the fuel passes therethrough quite rapidly.

The jets produced by nozzles 14, though they soon lose their directionalforce by the baffling action of the formation, have the desirableinitial effect of physically shattering any drill stem mud which mayhave collected directly around the well bore. This is in addition to theeffects produced by the heat and internal pressure of the gases. Thejets also provide an initial horizontally radial direction to thefracturing action to be described, which may be very important if thefonnation is shallow and, as very commonly occurs, is disposed closelyabove a water table, since in that case a downward initial direction ofthe fracturing action might break through intervening strata into thewater table to flood the well. The jets also assist in providing deeppenetration of the burning gases into the formation, as the fracturingaction progresses to greater distances from the well shaft. The equalangular spacing of nozzles 14 balances the reactive forces of the jetson chamber 11, so that said chamber is not battered against the walls ofthe formation.

As the fuel penetrates into formation 4 and burns rapidly, it producesintense heat and large quantities of gases, which due to theirconfinement cause a very high pressure. As previously discussed, thisaction is not as rapid or violent as that of a highorder explosive suchas nitroglycerine, producing at most an enabled to reach deeper into theformation through the fissures opened by the previous burn, and so onthrough a large number of burns until the formation has been fracturedto almost any desired distance from the well shaft. Thus each successiveburn operates on a larger face of the formation at a greater distancefrom the well shaft, and requires a greater amount of fuel to providethe desired fracturing pressure, while the initial burns operate only ina relatively restricted zone of the formation directly adjacent the wellshaft, and require correspondingly small amounts of fuel to produce thedesired pressures. It is for this reason that it is generally desirablethat the fuel supply rate be gradually increased as the treatmentprogresses.

It is also desirable that the gas pressures generated by each fuel burnbe allowed to dissipate before the next burn, in order to avoid thecreation of excessive demolition of the formation and destruction ofadjoining strata, which may separate the formation from water tables,and to avoid the build-up of excessive temperatures in the formation.Since the rate of pressure dissipation is determined by the permeabilityof the formation, which of course varies, it is desirable that the fuelpulsing rate be adjustable, either by changing the speed of motor 74 orotherwise. The entire treatment, including the amount of fuel required,its pulsing frequency and the amount of fuel delivered in each pulse,may be programmed in accordance with known characteristics of eachindividual well.

During the entire treatment, the fissures opened in the formation by thefracturing action of the fuel burns admits the intense heat of theexpansive gases deeper and deeper into the formation. This heat tends tomelt, or burn, any of sludges, tars, or paraffins, expanded clays, etc.,which are capable of being melted or burned, and which have theretoforeclogged original flow passages of the formation to reduce itspermeability. This tends to open some of said original flow passages,and hence to further increase the permeability of the formation.

The method and apparatus just described have many advantages overprevious methods and apparatus. In previous methods involving theinjection of hydraulic fluid into the formation, or the circulation ofacid or other chemicals, the circulation is greatly inhibited and slowedby the very loss of flow permeability they are designed to combat; whilein the present method the fracturing forms fissures which not onlyprovide new flow passages, but also provide for rapid and deeppenetration of the hot gases into the formation. The present fracturingmethod has several advantages over earlier methods wherein a charge ofhigh-order explosive such as nitroglycerine is lowered into the well anddetonated within the formation.

First, the fracturing performed by the present method is relativelygentle as compared to that produced by a high-order explosive. Thelatter produces masses of fine or pulverized debris, whichmust becarefully, and at substantial expense, cleaned out of the well beforethe well can be returned to service if the treatment is to be fullyeffective. The present method, wherein the fracturing is accomplished bya repetitive series of small low-order explosions or burns atsuccessively greater distances from the well bore, produces relativelylittle debris, so that the cleaning process is rendered much simpler andmore economical.

Second, since the series of burns in the present process can becontinued as long as desired, and since the distance of fracturing fromthe well shaft progressively increases, the formation can be effectivelyfractured to greater distances from the well shaft than is possible witha high explosive, even a heavy charge thereof.

Third, in the present method the fracturing is confined largely to theoil-bearing formation itself, not extending to any significant degree tothe adjacent ground strata. This effect results from the facts that thefuel burns produce only very low-order explosions, and that theresulting expanding gases naturally tend to seek the path of leastresistance, which is virtually always the oil-bearing formation itselfdue to its permeability.

Fourth, since the fracturing action is relatively gentle and confinedlargely to the formation itself, as described above, the present methodinvolves little risk that the adjacent ground strata will break toconnect the well to adjoining water 5 reservoirs and flood the well. Thedanger of this occurrence when using high explosives is quitesubstantial, especially in areas where the oil-bearing formations areshallow, and have water tables disposed closely thereto.

It should be noted that the present method and apparatus is effectivealso in tapping formations which have a naturally low flow permeability.Formations frequently occur which are porous and which contain largeamounts of oil in the pores thereof, but in which the pores or cellsthereof are poorly joined, if at all, by interconnecting flow passages,so that oil cannot flow, or can flow only very slowly, to a well shaft.The fracturing of the present method can open these formations to obtainheretofore unavailable quantities of oil. The present method andapparatus is also effective in treating gas wells, not so much in theremoval of clogging masses, but in the improvement of naturally lowpermeability.

In some cases, it may be possible that the fracturing action of themethod is ample to produce the desired results, and that the intenseheats produced by the burning of combustible fuel might not be desired.This might be the case, for example, where the characteristics of thewell, or more particularly of the elements clogging the formationthereof, are such that intense heat might cause said clogging elementsto tend to bake or fire into solid masses which would only clog theformation more severely. In such cases,chamber 11, which is in effect agas generator" in any event, could constitute a reactor chamber whereingases are produced, with the necessary explosive force, by theintermixture of appropriate chemicals therein, but without substantialproduction of heat. The chemicals could be fed downwardly through stem10 (in separate conduits of course) and injected into chamber 11 in thesame manner as the combustible fuel already described. Chamber 1] wouldthen become a cold gas generator."

While I have shown and described a specific embodiment of my method andapparatus, it will be readily apparent that they are subject to manyminor changes and modifications without departing from the spirit of theinvention.

What I claim as new and desire to protect by Letters Patent IS. 1. Amethod of stimulating the flow of an oil well consisting of thesuccessive steps of:

a. sealing off a portion of the shaft of the well within the oilbearingformation thereof,

b. creating gases within said sealed-off portion of the shaft, wherebysaid gases penetrate said formation, said gases being created with anexplosive force of low order whereby they fracture only that portion ofthe oil-bearing formation at and directly adjacent the zone of theformation penetrated by said gases, and are created in a series ofpulses or bursts, whereby the gases of each successive pulse areprovided access to progressively more distant zones of the formation bythe fracture fissures opened in the formation by the next precedingpulse, the amount of gas released in each successive pulse beingprogressively increased to provide the desired fracturing pressure inthe larger, more distant zone of the formation penetrated thereby.

2. A method as recited in claim ll wherein said gas pulses are spacedapart sufficiently to allow the pressure generated by 65 each pulse tobe dissipated before the next pulse occurs.

3. A method as recited in claim 1 wherein said gases are created by thesuccessive steps of:

a. injecting a quantity of combustible fuel into the sealed-off portionof said well shaft, and

b. igniting said fuel, said fuel, and the amount thereof introduced tosaid shaft, being such that said gases are produced with an explosiveforce of low order, capable of fracturing said formation but only thoseportions thereof at and directly adjacent the zone of the formationpenetrated by said gases, said fuel being injected in a sespaced apartsufficiently to allow the pressure generated by combustion of each pulseto be dissipated before the next pulse occurs.

5. An apparatus for stimulating the flow of an oil well comprising:

a. a chamber having means adapting it to be lowered through the shaft.of the well into horizontally aligned relation with the oil-bearingformation of the well,

b. means for sealing the well shaft at a point above said chamber, and

c. gas generating means carried in said chamber and operable to generatelarge quantities of gas under high pressure in a series of distinctpulses, and including means for adjusting the volume of gases generatedin each pulse, said chamber having outlets permitting the escape of saidgases from said chamber into said formation.

6. An apparatus as recited in claim 5 wherein said gas generating meansincludes means operable to adjust the frequency of said pulses.

7. An apparatus as recited in claim 5 wherein said gas generating meanscomprises:

a. means for injecting a combustible fuel into said chamber in a seriesof distinct pulses,

b. means operable to adjust the quantity of fuel injected in each pulse,and

c. means operable to ignite each pulse of said fuel.

8. An apparatus as recited in claim 7 with the addition of meansoperable to adjust the frequency of said fuel pulses.

1. A method of stimulating the flow of an oil well consisting of thesuccessive steps of: a. sealing off a portion of the shaft of the wellwithin the oil-bearing formation thereof, b. creating gases within saidsealed-off portion of the shaft, whereby said gases penetrate saidformation, said gases being created with an explosive force of low orderwhereby they fracture only that portion of the oil-bearing formation atand directly adjacent the zone of the formation penetrated by saidgases, and are created in a series of pulses or bursts, whereby thegases of each successive pulse are provided access to progressively moredistant zones of the formation by the fracture fissures opened in theformation by the next preceding pulse, the amount of gas released ineach successive pulse being progressively increased to provide thedesired fracturing pressure in the larger, more distant zone of theformation penetrated thereby.
 2. A method as recited in claim 1 whereinsaid gas pulses are spaced apart sufficiently to allow the pressuregenerated by each pulse to be dissipated before the next pulse occurs.3. A method as recited in claim 1 wherein said gases are created by thesuccessive steps of: a. injecting a quantity of combustible fuel intothe sealed-off portion of said well shaft, and b. igniting said fuel,said fuel, and the amount thereof introduced to said shaft, being suchthat said gases are produced with an explosive force of low order,capable of fracturing said formation but only those portions thereof atand directly adjacent the zone of the formation penetrated by saidgases, said fuel being injected in a series of pulses or bursts, wherebythe gases generated by each successive pulse are provided access toprogressively more distant zones on the formation by the fracturefissures opened in the formation by gases generated by the nextpreceding fuel pulse, the quantity of fuel injected in each successivepulse being progressively increased to provide gases of the desiredfracturing pressure in the larger, more distant zones of the formationpenetrated thereby.
 4. A method as recited in claim 3 wherein the fuelpulses are spaced apart sufficiently to allow the pressure generated bycombustion of each pulse to be dissipated before the next pulse occurs.5. An apparaTus for stimulating the flow of an oil well comprising: a. achamber having means adapting it to be lowered through the shaft of thewell into horizontally aligned relation with the oil-bearing formationof the well, b. means for sealing the well shaft at a point above saidchamber, and c. gas generating means carried in said chamber andoperable to generate large quantities of gas under high pressure in aseries of distinct pulses, and including means for adjusting the volumeof gases generated in each pulse, said chamber having outlets permittingthe escape of said gases from said chamber into said formation.
 6. Anapparatus as recited in claim 5 wherein said gas generating meansincludes means operable to adjust the frequency of said pulses.
 7. Anapparatus as recited in claim 5 wherein said gas generating meanscomprises: a. means for injecting a combustible fuel into said chamberin a series of distinct pulses, b. means operable to adjust the quantityof fuel injected in each pulse, and c. means operable to ignite eachpulse of said fuel.
 8. An apparatus as recited in claim 7 with theaddition of means operable to adjust the frequency of said fuel pulses.